The present invention relates to a cold storage unit comprising a heat-insulating outer housing in which a highly heat-conductive inner housing is located, cold air being circulated between both housings so as to indirectly cool a storage chamber in the inner housing.
In a conventional cold storage unit of this kind, as disclosed in Japanese Laid-Open patent No. 4-184071, a highly heat-conductive inner housing defining therein a storage chamber, is provided in a heat-insulating outer housing with a cold air circulating space therebetween. A cooling device including a cryostat is provided in the cold air circulating space so as to circulate cold air through the cold air circulating space in order to cool the inner housing, thereby the storage space is cooled indirectly through the intermediary of the wall surface of the inner housing.
With this cold storage unit, agricultural products such as vegetables, fruits or fresh flowers stored therein are held at a low temperature without vapor being emitted from the products being absorbed by the circulated cold air, and accordingly, the products can be preserved for a long time, being prevented from being dried and deteriorated.
Further, the above-mentioned agricultural products are alive, and accordingly, generate ethylene gas due to metabolism. There has been a problem that if the metabolism increases the concentration of ethylene gas in the storage chamber, the deterioration rate of the products is accelerated. Accordingly, in the above-mentioned laid-open patent, plate members made of a porous material such as green tuff are attached to the side walls of the inner housing to adsorb the above-mentioned ethylene gas in the porous material, and further, to allow the same to permeate through the plate member into the cold air circulating space in order to restrain the products from deteriorating.
However, the plate members made of the porous material (green tuff) are heavy and fragile, and accordingly, they are not easily handled during the assembly or maintenance of the storage unit. Further, there has been such a problem that the degrees of adsorbability and permeability thereof are low, and accordingly, the reprocessing thereof should have been carried out every one or two years in order to expel adsorbed ethylene gas.